Re: [PATCH v2 4/5] migration/block-dirty-bitmap: fix bitmaps migration during mirror job

[Eric Blake <eblake@redhat.com>]

On 12/19/19 2:51 AM, Vladimir Sementsov-Ogievskiy wrote:
> Important thing for bitmap migration is to select destination block
> node to obtain the migrated bitmap.
> 
> Prepatch, on source we use bdrv_get_device_or_node_name() to identify
> the node, and on target we do bdrv_lookup_bs.
> bdrv_get_device_or_node_name() returns blk name only for direct
> children of blk. So, bitmaps of direct children of blks are migrated by
> blk name and others - by node name.
> 
> Libvirt currently is unprepared to bitmap migration by node-name,
> node-names are mostly auto-generated. So actually only migration by blk
> name works.

It depends on whether -blockdev is in use.  With -blockdev, libvirt 
should be supplying a node name everywhere, without, it is only device 
names.

> 
> Now, consider classic libvirt migrations assisted by mirror block job:
> mirror block job inserts filter, so our source is not a direct child of
> blk, and bitmaps are migrated by node-names. And this just don't work.

Does Max' work to improve seeing through filters fix this?

> 
> Let's fix it by allowing use blk-name even if some implicit filters are
> inserted.
> 
> Note, that we possibly want to allow explicit filters skipping too, but
> this is another story.
> 
> Note2: we, of course, can't skip filters and use blk name to migrate
> bitmaps in filtered node by blk name for this blk if these filters have
> named bitmaps which should be migrated.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1652424

That bug has been marked CLOSED in the meantime, but it appears to be 
only because libvirt is now using -blockdev rather than the older drive, 
while the problem affects drive usage.

> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   migration/block-dirty-bitmap.c | 39 +++++++++++++++++++++++++++++++++-
>   1 file changed, 38 insertions(+), 1 deletion(-)
> 
> 

Okay, after reading some more history on this project (the curse of 
coming up to speed after volunteering to become a co-maintainer), it 
looks like Max's idea replaces this patch altogether.  How much of the 
rest of the series is still important?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org